add a amo/mkt flag to allow reindexing both concurrently #880698

This fixes bug #880698.

A ReindexingManager has been added to lib.es.models.Reindexing to group all the reindexing logic.
Functions are still imported from lib.es.utils as a convenience and a central point, abstracting the ReindexingManager stuff.

This is based on the previous PR (to be merged): https://github.com/mozilla/zamboni/pull/1433, only the https://github.com/magopian/zamboni/commit/48cd83a7ed44c152f175aff485e6830b014b5504 commit is related to this issue.

There's a glitch in the matrix I can't figure: https://github.com/magopian/zamboni/commit/48cd83a7ed44c152f175aff485e6830b014b5504#diff-3824d5c71612e823bbe939d7ea8b3032R33

For some reason, if there's a "_mkt" in the test name, it just won't be detected/run when using AMO settings (but will when using MKT settings). Changing the name to something else (like "_market") will solve the issue, and all the tests will then run (even with the AMO settings).

If anyone has an idea, just let me know ;)